Hyaline cartilage is a specialized connective tissue that plays an important role in dissipating loads in joints. It provides a smooth, low friction gliding surface that has resilience and resistance to compression and shear forces. It is composed of terminally differentiated chondrocytes embedded within an extracellular matrix that is produced, elaborated and maintained by the cells themselves. The extracellular matrix of cartilage is composed of three classes of molecules. The first is highly cross-linked fibrils of triple helical type II collagens that interact with other cartilage specific collagens, including type II and XI collagens. The abundant large aggregating proteoglycan aggrecan as well as some small proteoglycans such as biglycan and decorin comprise the second class. These proteoglycans contain chondroitin sulfates as their glycosaminoglycans side chains. The third class of proteins are non-collagenous proteins including cartilage oligomeric matrix protein (COMP; also sometimes referred to as thrombospondin-5) and link protein. The proper composition and arrangement of the cartilage extracellular matrix are important for maintaining the proper amount of water and electrolyte in the matrix, thus conferring its mechanical properties.
Seemingly inert and able to tolerate a tremendous amount of physical stress, cartilage can be damaged by a variety of mechanical, chemical and microbiological agents, often resulting in pain, swelling, loss of motion, and eventually disabling arthritis. The biggest limitation of cartilage is that it is incapable of healing. This inherent inability to repair is due to its avascularity, the immobility of chondrocytes, and the limited ability of mature chondrocytes to proliferate and alter their synthetic pattern. Various methods of stimulating cartilage repair have been used with varying success. Major surgical methods for treating osteoarthritis include tissue transplantation, chondrocyte and mesenchymal stem cell transplantation, and transplantation of artificial matrices including collagen gels, carbon fiber pads, and porous polylactic acids and other biodegradable synthetic matrices. Because of the limited availability of cartilage tissue for transplantation, therapeutic efforts have been focused on the transplantation of cells and matrices in creating a regenerated tissue resembling articular cartilage in its structure and its biochemical and mechanical properties. However, despite numerous efforts by numerous research labs and companies, the outcome of these methods is generally mixed and not very satisfactory. It would be useful to develop a better method of cartilage repair.